In video coding systems, spatial and temporal redundancy is exploited using spatial and temporal prediction to reduce the information to be transmitted. The spatial and temporal prediction utilizes decoded pixels from the same picture and reference pictures respectively to form prediction for current pixels to be coded. In a conventional coding system, side information associated with spatial and temporal prediction may have to be transmitted, which will take up some bandwidth of the compressed video data. The transmission of motion vectors for temporal prediction may require a noticeable portion of the compressed video data, particularly in low-bitrate applications. To further reduce the bitrate associated with motion vectors, a technique called Motion Vector Prediction (MVP) has been used in the field of video coding in recent years. The MVP technique exploits the statistic redundancy among neighboring motion vectors spatially and temporally. In the rest of this document, MVP may sometimes denote “motion vector prediction” and sometimes denote “motion vector predictor” according to contexts.
In High-Efficiency Video Coding (HEVC) development, a technique named Advanced Motion Vector Prediction (AMVP) is currently being considered by the standard body. The AMVP technique uses explicit predictor signaling to indicate the MVP selected from a MVP candidate set. In HEVC test model version 2.0 (HM-2.0), the MVP candidate set of AMVP includes spatial MVPs as well as a temporal MVP, where the spatial MVPs include two MVPs selected from two respective neighboring groups of the current block. The temporal MVP is derived based on motion vectors from a respective area of a reference picture by mapping the current block from the current picture to the reference picture. The respective area, i.e., the co-located block, in the reference picture may not have the same block size (prediction unit (PU) size) as the current block. When the respective area uses smaller block sizes than the current block, one of the blocks in the co-located block is selected as a co-located reference block. In HM-2.0, the temporal predictor is associated with the center block of the respective area while the previous version of HM uses the above-left reference block of the co-located block. If the MV for the co-located reference block does not exist, the temporal MVP is not available. It is desirable to develop an MVP derivation scheme that can improve the availability of the MVP from the neighboring blocks. The improved MVP derivation scheme may result in smaller motion vector residues and, consequently, better coding efficiency. Furthermore, it is desirable that the MVP derivation scheme will allow the MVP candidate to be derived at the decoder based on decoded information so that no additional side information has to be transmitted.